An uncontrollable urge to do something. Irresistible impulse is not usually a defence in law and it will not afford a defence of insanity, unless it arises out of a disease of the mind as defined by the McNaghten Rules. When, however, an impulse is irresistible in that the body reacts in an instinctive way to it, there may be a defence of involuntary conduct. An irresistible impulse may also constitute diminished responsibility. See also provocation.